Such measuring inserts are known, for example, from Offenlegungsschrift DE 102007036693 A1.
Individual or multiple measuring inserts are inserted in metal or ceramic protective tubes, which, as medium contacting jackets, are exposed to process related, sometimes drastic, temperature changes, wherein moisture adsorptively bound to the ceramic sintered material can be released upon sudden temperature increase, this having disadvantageous effects directly on the temperature sensor element, which leads to the formation of condensate, which can result in corrosion phenomena in the protective tube.
According to the state of the art, for the manufacture of temperature measuring inserts, usually prefabricated small tube stock pieces composed of magnesia (MgO) or aluminum oxide, semi-finished parts are equipped with temperature sensors and lead wires and inserted in metal sleeves, wherein a sealing action is usually not achieved and disadvantageous condensation can affect the total inner space including the sensor leads, depending on warehouse conditions and humidity.
The adsorptive behavior of water on molecular sieves such as zeolites or silica gels can be characterized by the curve of the appropriate adsorption isotherm, wherein the amount of the adsorbate on the adsorbent can be presented, for example, as a function of the partial pressure at constant temperature.
The adsorption of water on pore forming phases, such as aluminum oxide, Al2O3, or similarly structured metal oxides, can lead to multilayered buildup, wherein the adsorption isotherms, with increasing steam partial pressure, first experience a flattening of the buildup, before rising greatly again upon the formation of subsequent layers, followed at still higher partial pressures by condensate formation.
A tightly bound H2O monolayer on an Al2O3 surface can have a very high adsorption enthalpy and, as a result, can only be removed by means of high temperature or high temperature vacuum treatments.
The temperature dependence of steam partial pressure, in the case of pressed ceramic parts of the type which can be present in a measuring insert, can be shown by adsorption isosteres; an adsorption isostere for steam shows steam partial pressure at constant volume rising exponentially as a function of temperature.
Ceramic materials, such as aluminum oxide or comparable metal oxides, which, depending on pretreatment, can be open pored and have open pored adsorption locations, only require moderate desorption enthalpies for weakly adsorbed water molecules in order to transfer the water back into the gaseous phase, wherein, in the case of the presence of supplementally present condensates, only the evaporation enthalpy is to be applied.
Thus, in the case of usual process related heating, a marked steam partial pressure can occur in the immediate vicinity of ceramic materials containing adsorbed moisture.
Moisture present can significantly influence the correct functioning of sensor elements; respectively, subsequent condensate formation following cooling can, for example, contribute to irreversible corrosion effects on temperature sensors or their leads.